A short answer may be just another question. “When you buy a land, why do you pay the government?”

Government is a legal authority. So we pay them. Who are these registrars to pay them?

While buying a land, you pay registrar and the registrar in turn pays it to government, ISW, when you buy a domain, you pay registrar and the registrar in turn pays ICANN is my guess. Any one who has a different thought can feel free to comment.